Oxygen Electrocatalysis and Interfacial Charge Transfer with Transition Metal (Oxy)hydroxides: Fundamentals and Applications in Solar Water Splitting

The continued prosperity of human civilization will require replacing fossil fuels with renewable, sustainable, and carbon-free energy sources. More energy hits the earth in the form of sunlight in one hour than civilization uses in one year. The critical science and technology challenge is to develop scalable low-cost methods to convert solar energy into fuel and electricity and to store that energy for times when the sun isn’t shining.
One approach to the conversion and storage of solar energy is the direct photoelectrochemical production of hydrogen and oxygen from water using sunlight as the energy input. Building efficient architectures for this process requires interfacing high-quality semiconductors that absorb sunlight with efficient electrocatalysts that facilitate the multi-electron H² and O² evolution reactions. I first present our identification and mechanistic studies of Ni-Fe oxyhydroxides, which we found are the fastest known water oxidation catalysts under basic conditions when compared to other oxide-based thin films using a quantitative electrochemical-microbalance approach.¹ In-situ electrical, photoelectron spectroscopy, x-ray diffraction, and electrochemical analyses on Ni1–xFexOOH films were used to conclude that the reaction mechanism relies on the local electronic structure of a Ni-O(H)-Fe active site.² Studies of related mixed-metal oxyhydroxide catalysts revealed new fundamental activity trends for oxygen electrocatalysis in alkaline media.³
I then describe efforts to understand interfacial electron transport between such electrocatalysts and bulk semiconductors using theory and simulation,⁴ as well as a new dual-electrode photoelectrochemistry technique.⁵ We find that electrolyte-permeable redox-active catalysts such as Ni-Fe oxyhydroxides form so-called “adaptive” junctions where the effective interfacial barrier height to electron transfer depends on the charge state of the catalyst. These results provide new insight to guide the interface design of efficient water-splitting photoelectrodes.⁶

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